Various U.S. patent documents disclose sensors for assisting in the administration of CPR. For example, U.S. Pat. No. 5,589,639 (D'Antonio et al.) discloses a force sensing system for a CPR device which generates an intelligible output signal corresponding to a force parameter. The CPR device utilizes a signal indicative of the force being applied to the recipient's chest.
U.S. Pat. No. 5,496,257 (Kelly) discloses an apparatus for assisting in the application of CPR. The device rests on the recipient's chest. Chest compression forces are monitored by the device in order to ascertain the rate of compression and blood flow. This information is actively provided to the rescuer to prompt proper administration of CPR.
Various devices are disclosed which assist in the timing of the application of CPR, including U.S. Pat. No. 5,626,618 (Ward et al.) and U.S. Pat. No. 4,863,385 (Pierce). The '618 patent discloses, among other things, an electrode combination for cardiac pacing and cardiac monitoring in association with a bladder for use in the patient's esophagus for improving artificial circulation as a result of CPR. The '385 patent discloses a CPR sequencer which comprises a compact, portable, computer-controlled device, which provides timing and sequence guidance for helping a rescuer in the application of CPR to a recipient.
Each year there are more than 300,000 victims of cardiac arrest. Current conventional techniques for CPR introduced in 1960 have had limited success both inside and outside of the hospital, with only about 15% survival rate. Accordingly, the importance of improving resuscitation techniques cannot be overestimated. In the majority of cardiac arrests, the arrest is due to ventricular fibrillation, which causes the heart to immediately stop pumping blood. To treat ventricular fibrillation, defibrillation is administered which involves the delivery of a high energy electric shock to the thorax to depolarize the myocardium, and to allow a perfusing rhythm to restart. If, however, more than a few minutes pass between the onset of ventricular fibrillation and the delivery of the first defibrillation shock, the heart may be so deprived of metabolic substrates that defibrillation is unsuccessful.
The role of CPR is to restore the flow of oxygenated blood to the heart, which may allow defibrillation to occur. A further role of CPR is to restore the flow of oxygenated blood to the brain, which may prevent brain damage until the heart can be restarted. Thus, CPR is critical in the treatment of a large number of patients who fail initial defibrillation, or who are not candidates for defibrillation.
Various studies show a strong correlation between restarting the heart and higher levels of coronary blood flow. To restart the heart, if initial defibrillation fails (or is not indicated), coronary flow must be provided. With well-performed CPR, together with the use of epinephrine, brain blood flow probably reaches 30-50% of normal. Myocardial blood flow is much more limited, however, in the range of 5-20% of normal. In patients, heart restarting has been shown to correlate with the pressure gradient between the aorta and the right atrium, obtained between compressions (i.e., the coronary perfusion pressure). CPR, when applied correctly, is designed to provide a sufficient amount of coronary perfusion pressure by applying a sufficient amount of chest compression force. Unfortunately, however, studies indicate that CPR is performed correctly only part of the time —approximately 50% of the time according to a study conducted on 885 patients. Hoeyweghen et al., “Quality and Efficacy of Bystander CPR,” Resuscitation 26 (1993), pp. 47-52. The same study showed that long-term survival, defined as being awake 14 days after CPR, was 16% in patients with correct CPR, but only 4% when CPR was performed with less chest compression (p<.05). Thus, properly administered CPR can increase survival rates.
Not only is the correct application of CPR critical to the survival of the CPR recipient, but when initial defibrillation is unsuccessful, or is not indicated, it can be essential that CPR be applied immediately. The sooner persons are resuscitated, the more likely they will survive long-term with preservation of neurologic function. When initial resuscitative efforts at the scene of an arrest fail to restore native cardiac function, it is often the practice to transport the patient to the hospital with the hope that better CPR can be performed under the supervision of a physician. A number of studies have shown, however, that it is quite rare for a patient who is not resuscitated in the field to be resuscitated in the hospital, and survive with meaningful neurologic function. Even invasive interventions used in hospitals, such as open chest cardiac massage, have failed to improve survival rates, probably due to irreversible organ damage produced by prolonged schema during transportation.
The American Heart Association (AHA) published guidelines specify that chest compression during CPR should be done at a rate of 80-100 compressions per minute at a depth of 1.5 to 2 inches. During CPR courses, instrumented mannequins are generally used that measure the amount of chest compression a student applies. It is then up to the student to apply similar chest compressions in an emergency situation, without feedback, relying only on the feel and look of the compressions. Since there is no feedback, and since relatively small changes in the amount of compression can affect perfusion pressure, it is not surprising that CPR is often performed incorrectly.
As described above, various types of devices have been provided to help give the rescuer administering CPR feedback. However, these devices do not measure chest displacement. Rather, they measure compression force as a result of the applied CPR. This is problematic since with clinical CPR there is considerable variation in the compliance of different patients' chests, such that similar compression forces produce substantially different chest displacements in different patients.
Gruben et al. disclose in their article entitled “Sternal Force-Displacement Relationship During Cardiopulmonary Resuscitation,” Journal of Biomedical Engineering, Volume 115 (May 1993), p. 195, the use of mechanical linkages incorporating position-sensing transducers to measure chest displacement during clinical CPR. However, this mechanism presents problems in general clinical environments, such as delays in setup and awkward handling.
While resuscitation is in progress, it is vital that physicians, paramedics, and other healthcare professionals administering CPR be continuously aware of changes in the patient's electrocardiogram (ECG), particularly the heart rhythm. An incorrect assessment of the heart rhythm can lead to administration of inappropriate therapy or withholding of appropriate therapy. The chest compressions associated with CPR, however, introduce artifacts in the measured ECG signal that make its interpretation difficult. The rather inadequate approach generally used to facilitate ECG interpretation during CPR is intermittent cessation of chest compressions to provide a period of artifact-free ECG acquisition. Problems occur with this approach. For one, there is a loss of hemodynamic support when chest compressions are stopped. In addition, the ECG remains difficult or impossible to interpret once chest compressions are resumed. Accordingly, sudden changes in rhythm may not be appreciated until after a substantial delay. In addition, since survival from cardiac arrest has been shown to be related to blood flow generated during CPR, and since interruption of chest compressions will reduce blood flow, survival may very well be compromised by these interruptions.
The outcome of CPR may be improved if there were a means for reducing the CPR-induced artifacts present in an ECG signal in a manner which would allow the correct interpretation of the ECG without interrupting chest compressions applied during CPR. E. Witherow has performed studies which demonstrate that CPR-induced artifacts are due primarily to changes in the half-cell potential of electrodes, caused by their mechanical disturbance. This was published in a thesis entitled A Study of the Noise in the ECG During CPR, M.S. thesis, the Johns Hopkins University (1993), the content of which is hereby expressly incorporated by reference herein in its entirety.
There is a need for compact, portable, and economic tools for monitoring CPR efforts, aiding in the correct administration of CPR, and otherwise increasing the success of resuscitation efforts, e.g., by removing CPR-induced artifacts from ECG signals so CPR does not need to be stopped in order to obtain a good ECG reading.